The current state of the art involves using processes that depend on listening devices, such as hydrophones, placed in an adjacent parallel well, or co-located around the well of interest. This method is commonly referred to as Micro Seismic Monitoring (MSM) and is typified by U.S. Pat. No. 7,872,944 and is incorporated herein by reference.
The MSM technique has several basic flaws. (a) The method is a secondary indicator of the actual fluid location and extent within the sub-surface formation. (b) The method does not indicate if the proppant has been successfully introduced into the sub-surface formation. (c) The method does not report any information relating to the quality of the fluid present or fractures in the formation, as might be defined by measurements of porosity and conductivity. (d) There are intricate procedures required to move and relocate the sensor arrays for each sub-surface depth at which the fracturing operation that occurs.
There is also a speculative patent application, Controlled Source Fracture Monitoring US 2010/0147512, that describes how the process might operate using CSEM but not the method or techniques required to produce usable imaging data.
The proposed method in US 2010/0147512 Cramer et al, has several basic flaws. (a) In Cramer et al, the method requires a proppant that is modified to enhance its electromagnetic (EM) properties. (b) The proposed method in Cramer et al, requires an active transmitter to use CSEM or a very long period of data collection in order to use the TM method. (c) There is a requirement in Cramer et al, to electrically conduct the signal to the formation through the tubing, casing, or a wireline tool. (d) There are practical implications in Cramer et al, related to the method of transferring or coupling the excitation pulse to the formation, that limit the method proposed to only operating before or after the fracturing operation occurs. In Cramer et al, the amount of power required will be significant and introducing this power during fracturing operations will be fraught with problems. The power will directly control the depth of investigation and this may prove to be a limitation. (f) In Cramer et al, there is no discussion of the process of tuning the antenna to the tubing and surrounding formation to ensure good coupling of the AC signal. It is likely that without the critical step of tuning the antenna to the surrounding formation, the antenna will not couple reliably with the formation, resulting in inconsistent results and possible transmitter output stage failure, due to reflected signal from the formation. (g) The references that are supposed to teach the examiner about certain aspects of the invention do not discuss the mathematical issues relating to the position and polarization of the antenna which must be known for the current data inversion techniques referenced, to achieve convergence. It is intrinsic to the method stated in Cramer et al, that this information will be unknown and dependent on the shape and size of the tubing and the area over which the proppant and fluid have migrated in the formation, during the fracturing operation.
The interferometric technique has been used generally in many unrelated areas such as Magnetic Resonance Imaging (“MRI”) for medical imaging, Astrophysics in Baseline Radio Astronomy Imaging, Synthetic Aperture Radar (“SAR”) to image the earth's surface, Ground Penetrating Radar (“GPR) and Interferometric SAR, (InSAR or IFSAR). Each of these systems is using distinct parts of the electromagnetic spectrum but none has been successfully applied to CSEM. In fact, most practitioners of the art expect interference fringes to contain many multiples of pi phase shift across an image; however, as will be discussed further below, this is not the case with the disclosed subject matter. There are a few patents that are relevant as follows: US 2009/0302849 A1 for describing a modeling approach to the problem using Greens Theorem. ELECTROMAGNETIC EXPLORATION IVAN VASCONCELOS et al, which is hereby incorporated by reference in its entirety. There are also several papers that discuss the interferometry techniques as it applies to CSEM these are: Seismic interferometry by crosscorrelation and multi-dimensional deconvolution: a systematic comparison by Wapenaar et al which is hereby incorporated by reference in its entirety, and 3D synthetic Aperture for Diffuse Fields by Knaak et al 3 Oct. 2012 which is hereby incorporated by reference in its entirety. It is important to note that the latter of these two papers limits its investigation to existing marine CSEM data 0.25 Hertz.
The techniques of interferometry are well understood at higher frequencies but have been given little attention at low frequencies.
Interferometry Synthetic Aperture Radar (IFSAR) is a known technique that uses signal processing to improve resolution beyond the limitation of physical antenna aperture—examples of this technique are described SAR Marine User's Manual by Samuel W McCandless Jr. and Christophe R Jackson, which is hereby incorporated by reference in its entirety. One of the issues associated with this technique is the phase unwrapping error due to the Doppler effect, as disclosed in BASIC PRINCIPLES AND CURRENT ISSUES OF SAR INTERFEROMETRY by Olaf Hellwich, which is hereby incorporated by reference in its entirety. In the IFSAR technique, fixed interval measurements are collected from a linear deployment of receivers and are used to ‘synthesize’ a very long antenna. IFSAR allows the possibility of using longer wavelengths and still achieving good resolution with antenna structures of reasonable size. Combined with phase data, theoretically any resolution may be obtained.
Recent advances in noise reduction and phase accuracy in multi component CSEM receivers allow for additional improvements that result in significant in field resolution improvements. Such methods and systems are disclosed in U.S. Pat. Pub. No. 2012/0010818 filed on Jul. 7, 2011 and which is hereby incorporated by reference and U.S. Provisional Pat. No. 61/648,305 filed on May 17, 2012 which is incorporated by reference.
The concept of Pseudo Random Binary (“PRB”) codes as used in spread spectrum wireless transmission schemes is well understood. A description of a proposed system for near surface investigation using PRB codes can be found in U.S. Pat Pub No. 2010/0102822 filed on Dec. 26, 2009 and is hereby incorporated by reference. The Publication No. 2010/0102822 contains several restrictions that prevent its operation in the application envisioned herein; the most important is that of using short PRB codes. Using short PRB codes severely limits the effective depth (e.g. less than 300 m). To reach useful depths (e.g. over 300 m to 3,000 m+), the code must be much longer and transmitted with much greater power. The paper: The development and applications of a wideband electromagnetic sounding system using a pseudo-noise source by P. M. Duncan et al presented to the SEG in 1980 describes a similar PRB system and is included herein in its entirety.
There are several problems with current CSEM techniques that limit the ability to interpret the images created. These problems occur because the current systems generate images with large spatial uncertainty. In an effort to alleviate this problem most practitioners of the current art use complicated signal processing methods that involve modeling and inversion with other data sets, such as seismic data, in order to constrain the results. Inevitably the method leads to imprecise and uncertain conclusions.